Does `int a = 0, b = a` have undefined behavior? [duplicate]

Question

The question title says it all: do declarations of the form int a = 0, b = a have undefined behavior?

Answer 1

No, this is well-defined. This is a declaration with two declarators, a and b. Each declarator has an initializer.

Each init-declarator in a declaration is analyzed separately as if it was in a declaration by itself.

That is, the line is treated like:

int a = 0;
int b = a;

Answer 2

No, there is no Undefined Behavior.

Per Paragraph 8/3 of the C++11 Standard:

Each init-declarator in a declaration is analyzed separately as if it was in a declaration by itself

Also, as footnote 97 specifies:

A declaration with several declarators is usually equivalent to the corresponding sequence of declarations each with a single declarator. That is

T D1, D2, ... Dn;

is usually(*) equivalent to

T D1; T D2; ... T Dn;

This means that a is initialized first, then b is initialized and assumes the value of a. Also notice that even if this was not the case, there has been quite a long debate on SO over whether this would or would not be UB, and some consensus has been reached on this not being UB.

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(*): As explained by Olaf Dietsche in the comments, the situations where this equivalence does not hold are mentioned later on in the same footnote.